During a number of years there has been a rapid development in power systems and the capacity requirements of these in turn require highly reliable relaying principles for protecting the system or components of the system in case of faults. These protection requirements apply to many parts of the power system such as for example transformer differential protection, motor differential protection, generator differential protection and busbar protection.
In this kind of protection system, the incoming and outgoing currents of a certain protection zone have been measured since these may be used to detect if a fault occurs within or outside the protection zone. In order to measure these currents, so called current transformers, or CT, are used, one on each incoming or outgoing line. Further each line is provided with a circuit breaker for breaking the line in case of a fault.
Digital protection systems have been developed to monitor a power system. These protection systems not only requires fast operation speed for heavy fault currents, but also need to be stable for external faults which are close to the protection zone. As there are a lot of different current transformers connected to the feed lines and there is no impedance to limit the fault current within the zone, it might be a very severe CT saturation condition in case of an external fault close to the CT. The very heavy CT saturation will produce an inaccurate current value and thus a wrong picture for the type of differential protection systems used currently. As a consequence, the differential protection might misoperate in case of an external fault and thereby trip for protection against a non-existent internal fault, especially for heavy CT saturation conditions.
A very difficult technical problem for this type of protection system is called simultaneous faults. This means that an internal fault occurs following an external fault and it is not possible to produce a trip signal with current differential protection methods because there are enormous crossing currents while the internal fault is taking place. The worst case occurs when the external fault current is equal to the internal fault current. In this case both fault currents share the source current and the differential current will have a large difference compared with the restrained current.
The modern low impedance differential protection algorithm used can be expressed as follows. If we suppose a passive connection point with N transmision lines, Id represents the differential current and Ir represents the restrained current among those lines.                               I          d                =                                                      ∑                              i                =                1                            N                        ⁢                                                  ⁢            Ii                                                        (        1        )                                          I          r                =                              ∑                          i              =              1                        N                    ⁢                                          ⁢                                  Ii                                                          (        2        )            Id−k×Ir>D(3)
In case of internal fault we have Id=Ir so that equation (3) can be confirmed if we set the proper k value (k<1) and D value. Equation (3) is known as percentage differential protection since it introduces the restrained current in order to make protection more stable for external faults.
In case of normal load or external faults, Id should be zero so that the equation (3) is not satisfied. As a consequence there will not be a trip signal issued according to Kirchhoff's first law. In reality, Id is still larger than zero for external fault cases during CT saturation period so that a misoperation will be produced during this time period.
The main technical problems for the algorithms used with digital differential protection systems is misoperation due to external faults close to the feeder CT's especially in case of different CT cores. In this case, the saturation of CT in the faulted line will produce inaccurate current values similar to an internal fault in the measuring circuits, that is, the differential current Id will be the same as the restrained current Ir during the CT saturation period when an external fault occurs.
In the case of busbar protection, a further drawback with some protection systems is that CT saturation is compensated in each bay of the system. This means that there could be a plurality of measuring devices for a large power system area, which is costly and ineffective.